Continuing interest exists for static dissipative and electrically conductive laminates for use in various environments, including static dissipative work surfaces and conductive flooring materials. Among the prior patents of interest are the U.S. Pat. Nos. to Economy et al 3,567,689; Meiser 3,650,821; Wilks et al 3,922,383; Cannady 4,540,624; Berbeco 4,454,199 and 4,455,350; Grosheim et al 4,472,474; Cannady et al 4,480,001; and Ungar et al 4,784,908 and 4,724,187. Of particular interest are the patents directed to the use of carbon black filled paper, noting the patents of Economy et al, Meiser, and Ungar et al.
The difference between the static electrical properties of static dissipative and conductive material is measured by the surface resistivity (in ohms/square). The Department of Defense defines the electrical properties as follows:
Anti-static : greater than 10.sup.9 PA0 Static dissipative: between 10.sup.6 and 10.sup.9 PA0 Conductive : less than 10.sup.6.
A static dissipative environment having a resistivity on the order of about 10.sup.6 to 10.sup.9 ohms/square is needed for a work surface for the assembly and repair of electronic components. Electronic components often pick up charges in dry air. When the component is placed on a surface, the charge is discharged, destroying or damaging the component. If the work surface is static dissipative and connected to a ground, the charge will leak off and the damage can be avoided.
Standard high pressure decorative laminates have a surface resistivity of about 10.sup.11 to 10.sup.13 ohms/square. If the work surface resistivity is too high an electrostatic discharge can occur, destroying the electrical components. If the work top resistivity is too low, less than 10.sup.6 ohms, it becomes a safety hazard for electrical shock as well as a source of damage to electronic components. No particular surface product is suitable for all static dissipative and conductive environments as different usages and different environments require different properties.
Static dissipative laminates prior to that of the Ungar et al U.S. Pat. No. 4,784,908 suffered from other disadvantages in addition to being too conductive or not conductive enough. Some laminates have an upper surface containing conductive particles or fibers for providing a conductive path from the upper surface of the laminate to the interior. This can result in dusting of conductive material from the surface of the laminate as it wears, which can cause electrical short circuits.
Another problem with such conventional static dissipative laminates is that the surface of the laminate tends to lose its electrical conductivity when the relative humidity drops. The resistivity of conventional static dissipative and conductive laminates ca change by several orders of magnitude between 50% and 15% relative humidity.
Some conventional static dissipative laminates also have a problem with field suppression. If the laminate is constructed of a highly conductive layer buried under a relatively non-conducting surface, when the charged object is placed on the surface, a field is induced. When the object is removed, the charge creates the type of static electricity hazard the laminate was supposed to avoid.
Other patents of interest, although these do not relate to high pressure laminates, are the U.S. Pat. Nos. to Rooklyn 4,525,398; O'Brien 4,579,902; Klein 4,590,120; Keough 4,623,594; Nowell et al 4,885,659; and Daimon et al 4,891,264. Of these, the patents to Daimon, Klein, Nowell and Rooklyn are directed to the use of stainless steel or aluminum foil or fibers as electro-conductive elements.
The Rooklyn U.S. Pat. No. 4,525,398 is directed to a conductive laminate product capable of dissipating static charges. The laminate is formed of a thin, hard top layer of plastic material, a thin layer of aluminum foil bonded therebeneath to the top layer and a layer of backing material secured to the metallic foil.
The Daimon U.S. Pat. No. 4,891,264 in essence shows an electroconductive thermoplastic resin sheet. The surface resistivity is in the range of 10.sup.3 to 10.sup.6 ohms/square. The surface is comprised of hot-melt-adhesive fibers and electroconductive fibers of diameters of 1-30 .mu.m which are irregularly entangled with each other and integrally melt-adhere-d into a thermoplastic resin film. The electroconductive fibers used includes metal compounds, carbon fibers, stainless steel fibers, and composite synthetic fibers.
The Klein U.S. Pat. No. 4,590,120 is directed to a rigid or semi-rigid static reducing floor mat. The top layer is comprised of a web of conductive fibers, such as staple fiber materials made from stainless steel or electrostatically metallic coated materials such as Badische's 901 filament or Sauquoit's X-Static which are chemically bonded to the surface in a partially conductive polymeric matrix, these fibers having diameters as small as 0.5 mils (12.8 .mu.m).
The Nowell U.S. Pat. No. 4,885,659 is directed to a static dissipative mat comprised of a thermoplastic polymer layer and an electrically conductive metallized layer, such as aluminum coated or glass fiber tissue material disposed in the thermoplastic layer. It is required that the tissue material be grounded with a ground wire.
The use of metallic fibers in the prior art has always been, insofar as is known, in non-aqueous environments where surface oxidation phenomena are minimized. Moreover, these prior art uses of metallic fibers have not incorporated the metallic fibers as part of a paper sheet formed by traditional paper making procedures involving the use of aqueous slurries of paper making fibers, e.g. wood pulp. Nor have such prior art uses of metallic fibers subjected such metallic fibers to the extreme temperature and pressure conditions commonly encountered in the manufacture of high pressure decorative laminates, e.g. 230.degree.-340.degree. F. at 800-1600 psi in the presence of an aqueous solution of the laminating resin for a time sufficient to consolidate the laminate and cure the laminating resins, e.g. about 25 minutes.